Metal base cookware induction heating apparatus having improved control circuit using infra-red temperature sensor

ABSTRACT

A metal based cookware induction heating apparatus having an improved power supply and control circuit comprised of a chopper inverter including a gate controlled power thyristor coupled to and supplying an induction heating coil with periodic energization currents. The gating circuit controlling operation of the chopper inverter is controlled by a temperature sensing arrangement that includes an infra-red heat sensor device arranged to view the metal-based cookware or other object whose temperature is to be controlled. A constant current amplifier is connected in series circuit relationship with the infra-red heat sensor device across a source of low voltage direct current excitation potential and means are provided for periodically interrupting the view of the infra-red heat sensor device to produce an output alternating current signal component whose amplitude is indicative of the temperature of the metal-based cookware or other object being heated. An alternating current amplifier including an adjustable temperature set resistor is coupled to the output from the infrared heat sensor device with the setting of the adjustable temperature set resistor serving to control the gain and establish the temperature level at which the output from the alternating current amplifier attains an operative level of magnitude. An output on-off control amplifier either in the form of a Schmitt trigger type of circuit or reed relay is coupled to and controlled by the output from the alternating current amplifier for producing an amplified output on-off control signal representative of the temperature of the metal-based cookware or other object whose temperture is being controlled in accordance with the temperature setting of the adjustable temperature set resistor. The on-off output from the temperature sensing arrangement control an inhibit circuit connected to and controlling a zero point sensing and control circuit that in turn controls the operation of a soft starting zero point switching device connected intermediate the rectified ac supply terminals and the thyristor switching device employed in the chopper inverter whereby both radio frequency interference effects and audible noise production by the inductive heating coil during energization periods from the chopper inverter is substantially reduced due to the fact that energization of the coil under the control of the temperature sensing arrangement is inhibited until only the beginning or near the beginning of each half cycle of the rectified full wave potential appearing at the output from the high voltage rectifier. To further reduce RFI effects, a smoothing inductor having an inductance L3 and a smoothing capacitor having a capacitance C3 is connected in series circuit relationship across at least one of the commutating components of the chopper inverter power supply circuit. The combined impedance of the commutating capacitor, the smoothing inductor L3 and the smoOthing capacitor C3 is capactive in nature and is series resonant with the communtating inductor L1 to establish the commutating period t1. The smoothing inductor and the capacitor serve to shape the output current flowing through the smoothing inductor which is used as the inductive heating coil to substantially a sinusoidal waveshape having little or no radio frequency interference emission effects. Additionally, a capacitor switching arrangement is provided for the commutating capacitor C1 which is formed from a plurality of parallel connected capacitors and power rate controlling switching means for switching selected ones of the parallel connected capacitors into effective circuit relationship in the chopper inverter circuit in accordance with a desired rate of power production. Where the improved induction heating apparatus is employed in connection with metal-based cookware, the inductive heating coil preferably is in the form of a helically-shaped planar inductive heating coil and is disposed under a heat transparent member such as an infra red transmissive pyroceramic that can form a smooth continuous tabletop over the induction heating unit and itself does not become heated except by conduction and convection from the metal-based cookware. The inductive heating coil excites and heats the metal-based cookware directly so that all heat is generated only by the metal-based cookware and no excess waste heat is produced in the manner of known resistance heating units. Since the temperature sensing units directly views the bottom of the metal-based cookware through the heat transparent member and controls the application of power to the induction heating coil, temperature control is directly related to the actual temperature of the pan or other object being heated by the induction heating apparatus. As a consequence, a considerable improvement in efficiency and reduction of waste heat results. Temperature control is accurate, rapid in response and there is no overshout in temperature such as is characteristic of temperature control systems which control the temperature of a heat source which is external to the object being heated. Improved arrangements for the heat transparent member, infra-red heat detector, chopper device, and cooling fan together with preferred designs for the inductive heating coil with ferrite core of without to maximize coupling to the metal-base cookware or other object being heated, are described whereby further improvements in efficiency of operation, economy of components and minimum size are achieved.

United States Patent Peters, Jr. June 3, 1975 METAL BASE COOKWARE INDUCTION [57] ABSTRACT HEATING APPARATUS HAVING A metal based cookware induction heating apparatus IMPROVED CONTROL CIRCUIT USING having an improved power supply and control circuit INFRA-RED TEMPERATURE SENSOR comprised of a chopper inverter including a gate con- [75] Inventor: Philip H. Peters, Jr., Greenwich, power .thyristpr i q and l l an NY' tnduct1on heating COll with periodic energization currents. [73] Asslgnee: Envlronment/One Corp" The gating circuit controlling operation of the Schenectady chopper inverter is controlled by a temperature [22] Filed: June 9,1972 sensing arrangement that includes an infra-red heat sensor device arranged to view the metal-based [21] Appl 261327 cookware or other object whose temperature is to be Related US. Application Data controlled. A constant current amplifier is connected [62 Division f [31,643Ami16 1971, p NO- in series circuit relationship with the infra-red heat 3,7l0,062. sensor device across a source of low voltage direct current excitation potential and means are provided [52] U.S. Cl 219/10,:19; 73/355 R; 219/1077; for periodically interrupting the view of the infra-red 219/502; 250/338; 307/117 heat sensor device to produce an output alternating [51] Int. Cl. H05b 5/04 Current signal wmponent Whose amplitude is [58] Field or Search 219/1049, 10.77, 10.79, indicative of the temperature of the metal-based 219/502, 50l, 504, 507; 307/1 17; 250/338, Cookware or other object being heated. An alternating 340 342 347 351 353 359; 73 355 3 2 current amplifier including an adjustable temperature set resistor is coupled to the output from the infrared [56] Refefgnces Ci d heat sensor device with the setting of the adjustable UNITED STATES PATENTS temperature set resistor serving lto control I:heh gatiln and establish the temperature eve at w ie t e 32535 11332 Ell'lifi'ijjjjjj11111111111133 43%? Output m the alternating amplifier mains 31035I143 5/1962 Leatherman 1. 219 1077 9 mput 3309236 M967 Gunji D g I r I I v I 250/333 control amplifier either 1n the form of a Schmttt 3,356,908 12 1967 Mitchell 307/117 trigger yp of Circuit or feed relay is coupled to and 3,398,281 8/1968 Treharne 250/338 controlled by the output from the alternating current 3.417.619 12/1968 Francis 73/362 amplifier for producing an amplified output on-off .5 3.l8 /1970 hillips 3 control signal representative of the temperature of the 3,613,454 10/1971 McFadin. 73/2 metal-based cookware or other object whose 31637985 H1972 Stacey-M 2l9/5Ol UX temperture is being controlled in accordance with the ggzf temperature setting of the adjustable temperature set Primary ExaminerBruce A. Reynolds Attorney, Agent, or Firm-Charles W. Helzer resistor.

I3 Claims, 41 Drawing Figures 15 R m f f L E l :2 r9 l C l g 1 N I I l T 1 EATING INVERTER I I l PllSE s3 p F cowunnm;

1 l I r Ralp 25m 1 AND l rmossa PULSE I QC. SUPPLY GENERQTOR cmbmm I 34 I /\J 3o 23 22 P l f .l on SUPPLY snmuw zcno Pom FOR SENSOR 0cm swncmus a AMPLIFlER mman CONTROL L I 2 l l l l e mm- I6 25 warms 1 RATE comm.

2s raven/nun: l season no AMPLIFIER 132 SHEET IR SENSOR OUTPUT FIG 3 FIG 3B PATENTEDJUH3 I975 SHEET 7 3,881,781

INVERSE GAIN WITH soo- DIAL 551 FIG 4C DIRECT emu mm 200- DIAL SET 0 160 EAR LSET 1 L2 l5 r '1 l l7 l8 C1 E A SCR 0 ,"ATFHZ'YF 18 a w 3,887,781

SHEET 9 11 ye, F VF F scR f n L i V c FIG. 7A FL FIG. 7B

ItEBITBl PiTE TFPJY*F HF SHEET m Qk 2 N. I E Sm u 2 N I] Nb +11 J in. nu u u T NW: o a Q J CCCCIIWIAV 4 2 PATENTED SHEET SHEET PATENTED N 3 SHEET iNEUTRAL LINE T0 NEUTRAL T A NEU R L c TEMP. SENSOR AMPLIFIER SUPPLY FEW. SENSOR FIG. 136

FIG 14 METAL BASE COOKWARE INDUCTION HEATING APPARATUS HAVING IMPROVED CONTROL CIRCUIT USING INFRA-RED TEMPERATURE SENSOR This is a division of application Ser. No. 131,648, filed Apr. 6, 1971, now US Pat. No. 3,7l0,062.

BACKGROUND OF INVENTION lv Field Of Invention This invention relates primarily to a new and improved induction heating apparatus designed for use with metal-based cookware.

More particularly, the invention relates to a new and improved induction heating apparatus employing novel chopper inverter power supply circuits having im proved operating characteristics and novel control gating circuit arrangements using new and improved infrared heat sensing and coil arrangements for directly sensing and controlling the temperature of metal-based cookware or other objects being heated with an induction heating coil. The improved induction heating apparatus provides rapid, safe, clean and efficient heating which can be accurately controlled in a manner which minimized the production and loss of waste heat, and accomplishes this with compact equipment of relatively low cost and not too complex design.

2. Prior Art Situation Heretofore electrically operated home cooking appliances have employed resistance heating almost universally to convert electric into heat energy for home cooking purposes. While there are exceptions, such as are exemplified by the micro-wave heating ovens, for the most part, known electrical heating appliances use resistance heating. With such appliances the heat generated is proportional to PR where I is the current supplied to the resistance heating element and R is the resistance of the heating element. By switching or otherwise controlling the values of R and/or controlling l, the heat generated by a resistance heating unit can be controlled comparatively easily dependant upon the degree of control desired. However, with all resistance heating units the heat generated is controlled by controlling in some manner the electrical energy supplied to the unit. By prior calibration, the amount of heat generated by the resistance heating unit, and hence the resulting temperature of the object being heated are somewhat and only roughly related.

Consider, for example, the well known helically coiled. flat top electric stove heating units of the Calrod" type. These units are controlled by calibrated switches on the stove which at a given setting produce a measurable quantity of FR generated heat that then is thermally coupled to the object being heated such as a pan. A great deal of the generated heat is wasted and contributes to thermal pollution of the environment in which the stove is located. If the stove is located in an air conditioned kitchen, not only does the waste heat contribute to inefficiency, but also the additional cooling load imposed on the air conditioner by the waste heat contributes to further inefficiency. Over a considerable operating period such as the summer months of June through August, the accumulated inefficiencies can contribute to a significant wasteage in power and increased utility bills not to mention the added discomfort in the kitchen during the cooking periods and increased loading on the utility system during periods when predictably power shortages are most likely to occur.

In addition to the inefficiencies mentioned above, presently available electric l'-R cooking units provide no reliable control over the temperature of the pan or other objects being heated. This lack of reliable control is due to the differences in the transfer of heat to pans fabricated from different metals such as aluminum, copper, iron or alloys of metal, and the difficulty of relating the efficiency of thermal coupling to the several different types of cookware and calibrated settings of the control knob for adjusting heat generation by the resistance heating units. Hence, at best the calibrated settings at the control knobs are only an educated guess at what is the actual temperature of the pan or other object being heated. To overcome the above discussed and other deficiencies inherent in electrical resistance heating apparatus, the present invention was devised.

SUMMARY OF INVENTION It is therefore a primary object of the invention to provide a new and improved induction heating appara tus for use with metal-base cookware and other similar objects to be heated, which is compact, relatively low cost and simple in design and provides safe, clean and highly efficient heating.

Another object of the invention is to provide an induction heating apparatus of the above type which is capable of directly sensing and controlling the temperature of the cookware or other object being heated.

A further object of the invention is to provide an improved chopper-inverter power supply circuit for use primarily in induction heating equipment and which is reliable and safe in operation over a wide range of load ing.

A still further object of the invention is the provision of improved induction heating coil designs and temperature sensing arrangements for use in induction heating apparatus having the characteristics noted above.

In practicing the invention, an improved power sup' ply and control circuit arrangement for induction heating apparatus is provided and includes an improved chopper-inverter power supply circuit under the control of a gate controlled power thyristor such as an SCR that is coupled to and supplies an induction heating load with periodic energization currents. Full wave rectifier means are provided for supplying a rectified, un filtered direct current high voltage excitation potential to the chopper inverter circuit which is gated on periodically by an improved gating circuit means. One improved gating circuit means includes a timing circuit supplied from the voltage across the SCR which operates a voltage responsive switch such as a SUS or PUT that in turn gates on momentarily a constant current, a high voltage switching transistor connected in series circuit relationship with the primary winding of a pulse transformer across a filtered dc voltage supply obtained from the high voltage rectifier. The switching transistor amplifies the input from the timing circuit; and the amplified signal appearing at the secondary terminals of the pulse transformer is fed to the control gate of the large power rated thyristor comprising a part of the chopper inverter power supply circuit.

The improved chopper-inverter power supply circuit includes power supply terminal means for connection to a source of electric energy for energizing the chopper inverter. A filter inductor having an inductance L and a bidirectional conducting gate controlled semiconductor thyristor such as an SCR and a parallel diode are connected in series circuit relationship across the power supply terminal means with the filter inductor being interposed between the thyristor switching device and the supply terminal means A commutating inductor having an inductance L and a commutating capacitor having a capacitance C are connected in series circuit relationship across the thyristor switching device and are tuned to series resonance at a desired commutating frequency that provides a combined thyristor conduction and commutating period during each cycle of operation. Means for deriving output power are coupled across at least one of the commutating components and gating circuit means are coupled to the control gate of the thyristor switching device for rendering the device conductive at a controlled frequency of operation that provides an operating period T for the chopper inverter including a quiescent charging period in each cycle of operation where T r 1 such that the value (9 1;, equals substantially rr/Z radians and where 01 V C whereby the reapplied for ward voltage across the semiconductor switching de vice following each conduction interval is substantially independent of load. For improved operation, a smoothing inductor L and a smoothing capacitor C are connected in series circuit relationship across the commutating capacitor of the chopper inverter circuit and serve to shape the output current supplied by the chopper inverter to substantially a sinusoidal waveshape, waveshape having little or no radio frequency interference emission effects. With such an arrange ment. the smoothing inductor may comprise an induo tive heating coil. and the load is coupled to and derives output power from the circuit across the smoothing inductor. Further, a greater level of power is coupled to the load for given levels of peak voltage. and current on the SCR than is available when power is taken inductively from the commutating inductor.

To further reduce radio frequency interference emission effects (RFI) a soft starting zero point energization switching means may be connected in the circuit intermediate the output from the rectifier and the chopper inverter power supply circuit Zero point sensing and turn-on circuit means are coupied to and control the soft start zero point switching means for switching the zero point switching means on only at or near the be ginning of each half cycle of the rectified unfiltered unidirectional excitation potential appearing at the output of the high voltage rectifier means. Inhibit control means for inhibiting operation of the zero point sensing and turnon circuit means are provided and are controlled by a temperature sensing means for sensing the temperature of metal-based cookware or other objects being heated by the inductive heating coil. The temperature sensing means derives on-off control signals in response to the temperature dropping below or raising above a preset level and supplies the on-off controi signals to the inhibit control means for thereby controlling output power and hence heat generated in the metal base cookware or other objects by the inductive heat ing coil.

An improved temperature set and control circuit is provided which includes an infra-red heat sensor device arranged to view the bottom of the metal based cookware or other objects being heated directly The infra-red heat sensor is connected in series circuit relationship with a constant current source across a source of low voltage direct current excitation potential and means are provided for periodically interrupting the view of the infra-red heat sensor device to produce an output alternating current signal component whose amplitude is indicative of the temperature of the metal based cookware or other objects being heated. An alternating current amplifier is supplied from the output of the infra-red sensor device and includes an adjustable temperature set impedance for controlling the gain of the alternating current amplifier whereby the setting of the temperature set impedance serves to establish the temperature level at which the output from the alternating current amplifier attains an operative level of magnitude. The output from the alternating current amplifier then in turn controls an output on'off control amplifier in the form of either a Schmitt type of trigger or a reed relay which in turn is coupled to and controls the inhibit control means of the soft start zero point switching device which controls energization of the chopper inverter gating circuit means and the chopper inverter power supply circuit.

The temperature sensing and control arrangement preferably comprises a planar helically-shaped inductive coil having a central opening disposed under an insulating heat transparent supporting member or other similar arrangement such as an insulating support member having a heat transparent window upon which the metal-based cookware is supported. The infra-red heat detecting device is supported within a suitable heat reflector and views directly the bottom of the metal-based cookware through the central opening of the induction heating coil and the heat transparent supporting member for directly controlling the tempera ture of the cooking vessel. A light chopping device is disposed intermediate the heat transparent supporting member and the infra-red heat detector device for periodically interrupting the view of the infra-red heat detector to thereby produce the alternating current signal component and also preferably includes fan means for producing a flow of cooling air through the ambient atmosphere surrounding the heat sensing arrangement. Preferabiy also, a pan-shaped ferrite core member having a central opening is disposed around the inductive heating coil below the surface of the heat transparent supporting member for concentrating the lines of magnetic flux up into the bottom of the metal based cookware for further improving magnetic coupling to the inductively heated cooking vessel. Further improvement of magnetic coupling to the metalbased cookware can be obtained by appropriate design of the helically coiled, planar inductibe heating coil.

BRIEF DESCRlPTlON OF DRAWINGS These and other objects, features and many of the attendant advantages of this invention will be appreciated more readily as the same becomes better understood by reference to the following detailed description. when considered in connection with the accompanying drawings, wherein like parts in each of the several figures are identified by the same reference character, and wherein:

FIG. 1 is a functional block diagram of an improved power supply circuit and control for use with an induction heating apparatus constructed in accordance with the invention;

FIGS. 2, 2A and 2B illustrate different alternative forms of commutating circuit component arrangement usable with the chopper inverter power supply illustrated schematically in FIG. I;

FIGS. 3-3E illustrate different forms of inductive heating coil fabrication design, and a preferred manner of mounting the inductive heating coils with respect to a heat transparent insulating support for metal-based cookware to be inductively heated by the heating coil and also with relation to a heat sensing device chopper device, fan cooling arrangement. and ferrite core support for improved magnetic coupling to the pan or other metal-based cookware being inductively heated;

FIGS. 4 and 4A are detailed circuit diagrams of the improved power supply and control circuit for induc tion heating apparatus shown schematically in FIG. I; FIG. 4B is a schematic circuit diagram of a modified form of the temperature control circuit shown in FIG. 4A;

FIG. 4C is a temperature versus dial position characteristic curve for the temperature control circuits of FIGS. 4A and 48;

FIG. 5 is a partial circuit diagram of an alternative form of construction for the temperature sensing circuit arrangement usable with the system of FIG. 4;

FIG. 6 is a schematic circuit diagram of an improved power supply circuit according to the invention which employs alternating current excitation;

FIGS. 7-7H illustrate a series of waveforms typifying voltage and current relationships across several of the components of the power supply together with the frequency and reactance characteristics of an improved chopper-inverter power supply according to the inven tion;

FIG. 8 is a partial circuit diagram of an improved chopper-inverter of larger power rating for exciting two inductive heating coils stacked pancake fashion in the manner shown in FIG. 8A;

FIG. 9 is an alternative form of larger power rated chopper inverter similar to that of FIG. 8 but including the additional smoothing inductor and capacitor for reducing RFI effect;

FIG. 10 is a detailed schematic circuit diagram of an alternative the of chopper inverter power supply and control according to the invention;

FIG. 10A is a modification of the power supply shown in FIG. 10;

FIG. 11 is a detailed schematic circuit diagram of still another form of chopper inverter power supply constructed in accordance with the invention and illustrates a novel dv/dt control for reducing power output from the circuit upon a pan being removed with the circuit otherwise adjusted to produce full output power and heating;

FIG. 12 is a detailed schematic circuit diagram of still another form of the chopper inverter power supply circuit and pan-off control for reducing output power automatically upon a pan being removed from the circuit without first having cut back the output power setting;

FIGS. l3-I3C illustrate a low cost version of improved chopper-inverter power supply and temperature control according to the invention together with voltage waveforms that depict the operation thereof;

FIG. 14 is a circuit diagram of an over-temperature sensor and control useable with the power supply circuits of FIGS. 1-13; and

FIGS. 15 and l6-I6B illustrate different induction heating applications with which the improved induction heating apparatus, power supply circuit and control made available by the invention. can be used in ad dition to heating metal-based cookware.

DETAILED DESCRIPTION OF PREFERRED EMBODIMENTS FIG. 1 is a functional block diagram of an overall chopper inverter power supply and gating control circuit for use with induction heating apparatus con structed in accordance with the invention. The circuit of FIG. 1 is intended for use primarily in exciting an induction heating coil for heating metal-based cookware and will be so described in the following disclosure. However, the circuit of FIG. I and the several modifications thereof disclosed hereinafter also may be used with commercial induction heating apparatus as illus trated in connection with FIGS. 13 and I4. Hence. the invention is in no way restricted in application to use only for heating metal-based cookware in home appliances. Additionally. it might be noted that the several blocks or elements comprising the overall block diagram of FIG. I will be described in greater detail hereinafter in connection with other figures of the drawings such as FIG. 4, and hence the immediate following description will be of a general nature only to illustrate the broad construction and principles of operation of the system.

The circuit of FIG. I is intended to be energized from a conventional commercial or residential I 15 volt. l5-30 amp, 60 cycle/sec alternating current power supply connected to power supply terminal means II and I2 through a conventional fused plug or other outlet connection. It is preferred that a conventional cir cuit breaker 13 having a fast response time be included in the overall system for protection purposes although it may not always be required. The power supply terminal means 11 and 12 are connected to a full wave rectifier 14 of conventional construction which full wave rectifies the alternating current supply potential and supplies its output across a pair of high voltage direct current power supply busses or terminals 15 and 16. The output from the full wave rectifier 14 is unfiltered. and hence the potential appearing across the direct current power supply terminals 15 and 16 while unidirectional is in the form of a series of halfwave sinusoidal-shaped rectified high voltage pulses that drop substantially to zero value intermediate each halfwave pulse and have a frequency on the order of I20 cycleslsec or approximately double that of the alternating current supply connected to the supply terminal means [I and 12.

The full wave rectifier 14 supplies the excitation potential for a chopper-inverter circuit comprised by a filter inductance L a filter capacitor C a bidirectional conducting, gate controlled. semiconductor thyristor switching device formed by a power rated SCR I7 and reversely poled parallel connected, feedback diode 18. The SCR I7 and feedback diode I8 are connected across, and serve to excite at a relatively high frequency of the order of 20-30 kc/sec the chopper inverter circuit commutating components I9 to be described more fully hereinafter in connection with FIGS. 2-2B. A conventional R-C snubbing network for re ducing dv/dt effects on the SCR l7 and not shown in FIGS. I and 2-28. also may be connected across the 

1. A temperature set and control circuit including an infra-red heat sensor device arranged to view an object whose temperature is to be controlled, a constant current amplifier connected in series circuit relationship with the infra-red heat sensor device across a source of low voltage direct current excitation potential, means for periodically interrupting the view of the infra-red heat sensor device to produce an output alternating current signal component whose amplitude is indicative of the temperature of the object, an alternating current amplifier, an adjustable temperature set impedance coupled to and controlling the gain of said alternating current amplifier, the setting of said adjustable temperature set impedance serving to establish the temperature level at which the output from the alternating current amplifier attains an operative level of magnitude, output on-off control amplifier means coupled to and controlled by the output from said alternating current amplifier for producing an amplified output on-off control signal representative of the temperature of the object whose temperature is being controlled in accordance with the temperature setting of the adjustable temperature set impedance, and means for supplying operating bias potentials from a common reference point on the source of low voltage excitation potential supplying the alternating current amplifier and the on-off control amplifier means whereby the effect of variations in the excitation potential is minimized.
 1. A temperature set and control circuit including an infra-red heat sensor device arranged to view an object whose temperature is to be controlled, a constant current amplifier connected in series circuit relationship with the infra-red heat sensor device across a source of low voltage direct current excitation potential, means for periodically interrupting the view of the infra-red heat sensor device to produce an output alternating current signal component whose amplitude is indicative of the temperature of the object, an alternating current amplifier, an adjustable temperature set impedance coupled to and controlling the gain of said alternating current amplifier, the setting of said adjustable temperature set impedance serving to establish the temperature level at which the output from the alternating current amplifier attains an operative level of magnitude, output on-off control amplifier means coupled to and controlled by the output from said alternating current amplifier for producing an amplified output on-off control signal representative of the temperature of the object whose temperature is being controlled in accordance with the temperature setting of the adjustable temperature set impedance, and means for supplying operating bias potentials from a common reference point on the source of low voltage excitation potential supplying the alternating current amplifier and the on-off control amplifier means whereby the effect of variations in the excitation potential is minimized.
 2. A temperature set and control circuit according to claim 1 wherein the constant current amplifier comprises a field effect transistor having drain, source and gate electrodes and a series connected feedback resistor connected in series circuit relationship with the drain-source electrodes, the feedback resistor being connected through a feedback connection back to the gate of the field effect transistor for maintaining constant current thRough the field effect transistor and the series connected resistor, the series circuit comprised by the series connected feedback resistor and drain-source electrodes of said field effect transistor being connected in series circuit relationship with the infra-red heat sensor device.
 3. A temperature set and control circuit according to claim 2 wherein the alternating current amplifier comprises an integrated circuit operational amplifier having inverting and non-inverting input terminals and an output terminal with the output signal from the infra-red heat sensor device being AC coupled to the non-inverting input terminal, and the adjustable temperature set impedance comprises three variable resistors connected in series circuit relationship between the output terminal and the inverting input terminal with the outer most variable resistors serving as minimum and maximum temperature limit settings and the middle variable resistor serving to establish the desired operating temperature setting, and means for supplying the inverting and non-inverting input terminals with operating bias potentials from a common point on the source of low voltage excitation potential supplying the alternating current amplifier and infra-red sensor device.
 4. A temperature set and control circuit according to claim 3 wherein the output on-off control amplifier means comprises on-off bistable trigger circuit means responsive to the output from said alternating current amplifier, said bistable trigger circuit means having two stable states of operation and being triggerable from one stable state to the other in response to the output from the alternating current amplifier attaining a preset level of magnitude established by the temperature setting of the temperature set impedance and automatically reverting back to the first stable state upon the output dropping below the preset level of magnitude, and separate sources of low voltage direct current excitation potential are provided for the bistable trigger circuit means and for the infra-red heat sensor device and alternating current amplifier whereby improved isolation during operation between these portions of the circuit is obtained.
 5. A temperature set and control circuit according to claim 3 wherein the output on-off control amplifier means comprises a reed relay having its excitation winding connected in the output of the alternating current amplifier and a high voltage charging network connected to supply low voltage gating-on potentials to the control gate of a switching semiconductor device and having the armature of the reed relay connected to selectively discharge the charging network in accordance with the output derived from the alternating current amplifier.
 6. A temperature sensing and control signal deriving arrangement for a cooking vessel to be inductively heated comprising a solid infrared heat transparent insulating supporting member for a cooking vessel, and an infra-red heat detecting device viewing the cooking vessel directly through the solid heat transparent supporting member for deriving an output electric control signal that is directly related to the temperature of the cooking vessel.
 7. A temperature sensing and control signal deriving arrangement according to claim 6 wherein the heat transparent support member comprises an electrically insulating heat transparent member of material such as pyroceramic.
 8. A temperature sensing and control signal deriving arrangement according to claim 6 further including an infra-red light chopping device disposed intermediate the heat transparent supporting member and the infra-red heat detecting device to thereby produce an alternating current signal component in the output of the infra-red heat sensor device whose amplitude is indicative of the temperature of the cooking vessel.
 9. A temperature sensing and control signal deriving arrangement according to claim 8 wherein the lightchopping device also includes fan means for producing a flow of cooling mediuM through the ambient atmosphere surrounding the arrangement.
 10. A temperature sensing and control signal deriving arrangement according to claim 6 further including a heat reflector with the infra-red heat detecting device being supported within the heat reflector for concentrating the infra-red heat rays emitted from the cooking vessel upon the infra-red heat detecting device.
 11. A stabilized sensor circuit comprising a sensor device for sensing changes in a physical phenomenon being monitored and a constant current amplifier connected in series circuit relationship with the sensor device across a source of excitation potential, said constant current amplifier comprising a field effect transistor having drain, source and gate electrodes and a series connected feedback resistor connected in series circuit relationship with the drain-source electrodes, the feedback resistor being connected through a feedback connection back to the gate of the field effect transistor for maintaining constant current through the field effect transistor and the series connected feedback resistor, the series circuit comprised by the series connected feedback resistor and drain-source electrodes being connected in series circuit relationship with the sensor device.
 11. A stabilized sensor circuit comprising a sensor device for sensing changes in a physical phenomenon being monitored and a constant current amplifier connected in series circuit relationship with the sensor device across a source of excitation potential, said constant current amplifier comprising a field effect transistor having drain, source and gate electrodes and a series connected feedback resistor connected in series circuit relationship with the drain-source electrodes, the feedback resistor being connected through a feedback connection back to the gate of the field effect transistor for maintaining constant current through the field effect transistor and the series connected feedback resistor, the series circuit comprised by the series connected feedback resistor and drain-source electrodes being connected in series circuit relationship with the sensor device.
 12. A temperature sensing circuit including in combination a thermistor and adjustable resistor connected in series circuit relationship across a set of power supply terminals, a voltage dividing network connected across the power supply terminals in parallel with the series connected thermistor and adjustable resistor, a programmable unijunction transistor device having its anode and cathode connected in series circuit relationship with a load resistor between an intermediate point on the voltage dividing network and one of the power supply terminals, said programmable unijunction transistor device having a gate connected to the juncture of the thermistor and the adjustable resistor, a charging capacitor coupled between the gate and one of the power supply terminals, and means for deriving an over-temperature alarm signal from across said load resistor for supply to an over-temperature alarm indicating means. 